Intact virus particles of avian and also mammalian oncoviruses are being treated with dimethylsuberimidate and related bifunctional reagents. The protein-protein complexes as well as protein lipid complexes that are formed are being analyzed by SDS gel electrophoresis. Cleavage of avian gag precursor polypeptide Pr76 by virion protein p15 is being studied in vitro with the aim of elucidating the nature of the cleavage sites. Other gag-related polyproteins cleaved by p15 are also being analyzed.